If it has a POINTS set, then don't worry about the gap.
It doesn't matter as long as is does two things "cleanly" :
1. Closes fully
and
2. OPENS fully.
The spark occurs when the points snap OPEN. If the gap is set anywhere between 10 and 30 thou you'll be doing just fine. Remember, the gap changing also changes ignition TIMING.
People will rabbit on about dwell angles and so on, but just ignore them.
What matters is TIME ..... enough TIME for the windings in the coil to build up the magnetism that causes the spark when the points OPEN and the flow of current stops, causing the collapse and reverse of the magnetism, and that makes the spark.
As this can happen from about one rpm and any speed up to many thousands of RPM, it can be seen that it can happen just as well fast as slow, so even at high speed, the electricity has plenty of time to do the job.
The crucial thing is CLEAN POINTS, clean opening, and a really good CONDENSER (capacitor). Also, some COILS make heaps better sparks than others. I think Bosch coils make good sparks.
CharlesY